In draft pulping operations, where the goal is to remove lignin while retaining carbohydrates, yield is increased by minimizing carbohydrate (i.e., cellulose and hemicellulose) degradation. This degradation occurs through the “peeling” reaction in which sugar units are sequentially removed from the reducing end group of the polysaccharide chains. One way to prevent this reaction is to convert aldehyde groups on the wood polysaccharides to a form which is relatively inert to further “peeling”. This conversion is achieved by either oxidizing the aldehyde to its corresponding carboxylic acid (Alfredsson et al., 1963), (Holton, 1977) or, alternatively, reducing it to its alcohol form (Hartler, 1959), (Pettersson et al., 1961). The two methods that are applied in the pulp and paper industry involve the oxidation process and use anthraquinone (Holton, 1977), or polysulphide (Clayton et al., 1967), (Landmark et al., 1965), (Sanyer et al., 1964), (Teder, 1969), or both as oxidizing agents. Anthraquinone is a catalytic additive while polysulphide is generated from white liquor by oxidation of sodium sulphide in one of several processes (Dorris, 1992), (Smith et al., 1977).
Polysulphide has recently been found to be a mixture of active and inactive components named herein “active” and “inactive” polysulphide. The difference between these two forms is that active polysulphide is an oxidant that can oxidize carbohydrate aldehyde groups to acids which are no longer susceptible to the peeling reaction during kraft pulping. The result of using active polysulphide is that pulp yield is increased. In contrast, inactive polysulphide cannot oxidize carbohydrate aldehyde groups and does not cause a yield increase during pulping. Both active and inactive polysulphide are measured in the gravimetric test and both are observed in spectrophotometric analysis except at 416 nm; at 416 nm, only active polysulphide is measured. The most important measurement is the ratio of PS as measured by spectrophotometry (PS416) to PS as measured by gravimetry (PSGR). This number describes how much of the polysulphide is active; the closer the ratio is to 1 the greater is the activity of the liquor. Heat treatment of the liquor (van Heek et al., 2004) can convert inactive polysulphide into active polysulphide.